I. Field of the Invention
The present invention relates generally to ignition systems for spark ignition reciprocal piston internal combustion engines and, more particularly, to an electronic ignition system.
II. Description of the Prior Art
Reciprocal piston internal combustion engines comprise at least one cylinder in which a piston is reciprocally slidably mounted. The piston is connected by a connecting rod to a crank shaft so that reciprocation of the piston within the cylinder rotatably drives the crank shaft.
A combustible gaseous mixture is supplied to the cylinder during the intake stroke of the piston and this gaseous mixture is then compressed by the piston during the compression stroke. A spark plug is activated or energized at or near the end of the compression stroke which ignites the gaseous mixture. The expansion of the gaseous mixture following combustion drives the piston from the cylinder head and towards the crank shaft thus rotatably driving the crank shaft in the desired fashion. In a four cycle engine, the next outward reciprocation of the piston within the cylinder exhausts the combustion products from the cylinder and the above cycle is then repeated.
In order to provide the proper timing for activation of the spark plug, many of the previously known systems have employed a distributor having a distributor shaft which is rotatably mounted in synchronism with the engine crank shaft. A cam is mounted to the distributor shaft while electrical contact points cooperate with the cam. Thus, upon rotation of the distributor shaft, the points make and break an electrical contact for each desired activation of the spark plug or spark plugs for a multi-piston and cylinder engine.
When the electrical contacts make a connection, the primary coil of an ignition transformer is electrically connected to a source of electrical energy which temporarily stores electrical energy in a magnetic field. The secondary coil of the transformer is connected to the spark plug so that, when the points break the electrical connection between the primary coil and the power source, the energy stored in the primary coil induces current in the secondary coil which discharges through the spark plug thus activating or energizing the spark plug. When energized, the spark plug sparks and ignites the fuel mixture within the cylinder.
These previously known distributor ignition systems all suffer from a number of common disadvantages. One disadvantage of such systems is that the mechanical components are subject to wear and tear and require frequent maintenance and replacement. Such systems are also expensive in construction and are also heavy in weight thus rendering them undesirable for weight critical applications, such as aircraft engines.
There have, however, been a number of previously known electronic ignition systems which replace the previously known distributors, distributor cams, contact points and their associated components. The previously known electronic ignition systems typically comprise a microprocessor which receives an input signal from the engine representative of the engine crankshaft position. The microprocessor then calculates the optimal time for activation of the various spark plugs within the engine as a function of the crankshaft position. The microprocessor then generates output signals which turn on electronic switches to electrically connect the primary coil of the transformer to a power source prior to the desired activation of the spark plug. At the desired activation of the spark plug, the microprocessor opens the electronic switch so that the stored energy in the primary coil of the transformer discharges through the secondary coil into the spark plug thus activating the sparkplug in the desired fashion. The microprocessor is also typically programmed to advance the fuel ignition before top dead center as the engine speed increases.
One disadvantage of these previously known electronic ignition systems, however, is that the microprocessor utilizes the rotational position of the crank shaft to determine the point at which the primary coil of the ignition transformer is connected to the power source. Consequently, the duration of the "dwell" period, i.e. the period during which the primary coil of the ignition transformer is electrically connected to the power source, varies as a function of engine speed. At higher speeds, the dwell period is much shorter than at lower speeds. Consequently, in order to ensure that the primary coil of the transformer receives a sufficient amount of electrical power to fire the spark plug at high engine speeds, it is necessary that the dwell period encompasses a relatively large crank angle of the crank shaft. This in turn reults in an excessively large dwell period for low engine speeds which wastes electrical power by creating large current drains. Such large current drains require larger storage batteries thereby increasing the weight of the ignition system.
A still further disadvantage of these previously known electronic ignition systems is that, in many applications, the electrical power source does not remain constant but, rather, varies between an upper and a lower limit. Since these previously known systems employ only the rotational position of the crank shaft to begin the dwell period, a relatively low battery voltage results in an insufficient supply of electrical energy to the primary coil of the transformer, particularly at high engine speeds. Such low battery voltage can result in inadequate activation or energization of the spark plugs and can cause engine misfiring. Conversely, a relatively high voltage from the battery results in excessive electrical energy supplied to the primary coil of the transformer and wastes electrical energy.
A still further disadvantage of many of these previously known electronic ignition systems is that the spark advance is automatically varied by the ignition system as a function of engine speed. In many applications, such as aircraft engines, it is highly desirable to enable the operator to manually control the spark advance for the engine. While this is easily accomplished with mechanical distributor systems by merely rotating the distributor housing, the previously known electronic ignition systems have not included any means for allowing the operator to manually set the spark advance.